1. Field of the Invention
The present invention relates to an image information transmission system for transmitting image information.
2. Description of the Related Art
An electronic still video system is known as one conventional example of an image information transmission system for transmitting image information. The electronic still video system is arranged to record a still image signal on a magnetic disc which serves as a recording medium, and to reproduce a still image signal recorded on the magnetic disc.
Such an electronic still video system conforms to an existing television system such as the NTSC system or the PAL system, and it is difficult to record or reproduce an image of high resolution by the electronic still video system. However, in preparation for the advent of a high-definition television system such as that proposed by the Nippon Hoso Kyokai (NHK), a CHSV (Compatible High-Definition Still Video) system has already been proposed which can record and reproduce an image signal of a resolution of approximately 1,000 pixels in the vertical direction and approximately 1,300 pixels in the horizontal direction.
The CHSV system utilizes a technique called analog transmission of sample values for recording and reproducing an image signal. The analog transmission of sample values is intended to transmit correctly a sample-value sequence of time interval T by using an analog transmission path having a predetermined limited band.
Requirements for realizing the aforesaid analog transmission of sample values are as follows:
(1) It is necessary to meet the well-known Nyquist condition that the frequency characteristic of an analog transmission path has a linear phase and a symmetrical roll-off characteristic centered at a frequency of 1/2T.
(2) It is necessary to perform re-sampling on a reception side, i.e., during reproduction, in accordance with a time base which is not offset from that of sampling performed during recording.
The CHSV system is basically arranged to satisfy the aforesaid requirements and also to record and reproduce a sequence of sample values of an image signal in a recording format which conforms to the conventional electronic still video system, as shown in FIG. 1.
FIGS. 2(a), 2(b) and 2(c) are illustrations showing sampling positions on a luminance signal Y, a color-difference signal C1(R-Y) and a color-difference signal C2(B-Y), respectively. As shown in FIGS. 2(a), 2(b) and 2(c), respectively, the luminance signal Y is divided into four signals, YA (sample points on a (4n+0)-th line), YB (sample points on a (4n+1)-th line), YC (sample points on a (4n+2)-th line) and YD (sample points on a (4n+3)-th line); the color-difference signal C1(R-Y), into four signals, C1A (sample points on an (8n+0)-th line), C1B (sample points on an (8n+2)-th line), C1C (sample points on an (8n+4)-th line), and C1D (sample points on an (8n+6)-th line); and the color-difference signal C2(B-Y), into four signals, C2A (sample points on an (8n+1)-th line), C2B (sample points on an (8n+3)-th line), C2C (sample points on an (8n+5)-th line), and C2D (sample points on an (8n+7)-th line). The sample-value sequences of the thus-divided signals are recorded on four tracks on the magnetic disc in a predetermined combination.
A frequency-modulated luminance signal FM-Y and a frequency-modulated color-difference line-sequential signal FM-C are recorded on each of the four tracks on the magnetic disc in a frequency-multiplexed state as shown in FIG. 3. The combination of the frequency-modulated luminance signal FM-Y and the frequency-modulated color-difference line-sequential signal FM-C is, for example, such that the luminance signal YA and the color-difference line-sequential signal C1A/C2C are recorded on the first track, the luminance signal YB and the color-difference line-sequential signal C2A/C1C on the second track, the luminance signal YD and the color-difference line-sequential signal C2B/C1D on the third track, and the luminance signal YC and the color-difference line-sequential signal C1B/C2D on the fourth track.
Accordingly, track patterns on the magnetic disc on which the luminance signals and the color-difference line-sequential signals have been recorded in the above-described combination are as shown in FIG. 4.
It is to be noted that the high-definition image signal which has been recorded on each track on the magnetic disc in the above-described manner conforms to a conventional electronic still video format. Accordingly, field reproduction from each track is possible and frame reproduction is also possible from the second and third tracks.
In the case of the reproduction of the high-definition image signals which have been recorded on the four tracks in the above-described divided form, re-sampling is performed for positions identical to those of sampling performed during the recording. Re-sampled information is stored in a memory, and information on the sample points shown as X in FIGS. 2(a), 2(b) and 2(c) is interpolated by using information on the sample points shown as O. Thus, the high-definition still image signal of 1,000 pixels.times.1,300 pixels is restored.
In the CHSV system, since the sampled information is recorded and reproduced on and from the magnetic disc in the above-described manner, jitters occur in a rotating system for the magnetic disc and time base variations occur in a signal reproduced from the magnetic disc. As a result, the aforesaid requirement for realizing the analog transmission of sample values, i.e., the requirement that sampling positions for recording coincide with those for reproduction, is not satisfied.
To cope with this problem, at the time of recording by the conventional CHSV system, in each of horizontal blanking periods such as those shown in FIG. 5, a burst-like pilot signal for time base correction (TBC) is frequency-multiplexed onto a frequency band of 2-3 MHz in FIG. 3, i.e., a valley between the frequency band of the frequency-modulated luminance signal FM-Y and the frequency band of the frequency-modulated color-difference line-sequential signal FM-C, as viewed in FIG. 3. With respect to the luminance signal, one HIT (Horizontal Interval Test) pulse whose pulse width T corresponds to one sampling period is added in each horizontal scanning period as shown in FIG. 5. During reproduction by the conventional CHSV system, re-sampling clock signals which follow jitters occurring in the rotating system are generated by a PLL (Phase-Locked Loop) circuit by using the pilot signals for TBC which have been frequency-multiplexed during the recording and, in addition, phase matching of the resampling clock signals is performed by using the HIT pulses added during the recording. Information on the positions of sampling performed during the recording is obtained by performing re-sampling with the re-sampling clock signals.
If the stability of the electronic still video system and the influence of noise or the like are taken into account, a loop filter in the PLL circuit for forming the re-sampling clock signals which follow the jitters occurring in the rotating system will be designed to have a cutoff frequency of not higher than several hundred kilohertz. As a result, if jitters of frequency more than the cutoff frequency occur, it is impossible to maintain a correct sampling phase, thus resulting in the problem that no analog transmission of correct sample values can be performed.